Zeolitic materials having chabazite (CHA) framework structure are widely used in important actual technical areas such as in the automotive industry where the materials are employed as catalysts. The reduction of nitrogen oxides with ammonia to form nitrogen and H2O can be catalyzed by metal-promoted zeolites to take place preferentially to the oxidation of ammonia by the oxygen or to the formation of undesirable side products such as N2O, hence the process is often referred to as the “selective” catalytic reduction (“SCR”) of nitrogen oxides, and is sometimes referred to herein simply as the “SCR” process. The catalysts employed in the SCR process ideally should be able to retain good catalytic activity over the wide range of temperature conditions of use, for example, 200° C. to 600° C. or higher, under hydrothermal conditions and in the presence of sulfur compounds. High temperature and hydrothermal conditions are often encountered in practice, such as during the regeneration of the catalyzed soot filter, a component necessary for the aftertreatment of exhaust off-gas. Thus, these materials are of high economical and ecological interest. Due to the said technical areas and the resulting need of high amounts of the materials, there is an increasing demand for efficient processes for the preparation of these materials.
Molecular sieves are classified by the Structure Commission of the International Zeolite Association according to the rules of the IUPAC Commission on Zeolite Nomenclature. According to this classification, framework-type zeolites and other crystalline microporous molecular sieves, for which a structure has been established, are assigned a three letter code and are described in the Atlas of Zeolite Framework Types, 5th edition, Elsevier, London, England (2001). Chabazite is one of the molecular sieves for which a structure has been established, and the material of this framework-type is designated as CHA. Zeolitic materials as used herein are defined as metallosilicate frameworks including aluminosilicates, borosilicates and gallosilicates. It does not include the MeAPSO, APSO, or AlPO family of materials.
Chabazite is a zeolite which occurs in nature and also has synthetic forms. Synthetic forms are described in “Zeolite Molecular Sieves” by Breck (1973). The structure of Chabazite is described in “Atlas of Zeolite Structure Types” by Meier and Olson (1978). The Chabazite structure has been designated with the structure code, “CHA”.
Natural Chabazite exists in nature and has a SiO2:Al2O3 typically less than 10. Synthetic forms of this low SiO2:Al2O3 range include zeolite “K-G”, zeolite D and zeolite R. Zeolite “K-G” is reported by Barrer et al. in J. Chem. Soc., 1956, p 2892-. Zeolite D is reported in British patent number 868,846. Zeolite R is reported in U.S. Pat. No. 3,030,181.
Synthesis of high-silica Chabazite (>10 SiO2:Al2O3) is reported in U.S. Pat. No. 4,544,538, U.S. Pat. No. 6,709,644 and US 2003/0176751 A1.
U.S. Pat. No. 6,709,644 discloses a high-silica CuChabazite (SSZ-62) with small crystal size (<0.5 microns) with application in SCR of NOx.
WO 2008/106519 discloses a catalyst comprising: a zeolite having the CHA crystal structure and a mole ratio of silica to alumina greater than 15 and an atomic ratio of copper to aluminum exceeding 0.25. The catalyst is prepared via copper exchanging NH4+-form CHA with copper sulfate or copper acetate. Catalytic activity is largely retained after hydrothermal aging at 850° C. for 6 hours.
WO 2008/132452 discloses a number of zeolite materials, including CuSSZ-13, that can be loaded with iron and/or copper. Catalytic activity is largely retained after hydrothermal aging of CuSSZ-13 at 900° C. for 1 hour. Although no specific mention of Na levels appears it is stated than an ammonium exchange is employed prior to the Cu exchange to remove Na.
WO 2008/118434 indicates that a CuSSZ-13 (15 to 60 SiO2:Al2O3) material that can retain at least 80% of its surface area and micropore volume after hydrothermal aging at 900° C. in 10% steam for 1 to 16 hours would be suitable for application in SCR. Example 3 indicates that an ammonium exchange is carried out to remove residual Na. Additionally, a comparison of medium-sized crystals to large-sized crystals of SAPO-34 indicated improved stability for the larger crystals.
In all cases Na is first removed by ammonium exchange prior to the introduction of Cu. The resultant Na content is not disclosed. In table 8 of U.S. Pat. No. 4,544,538 Na contents of >0.5% Na2O are reported for examples 2 through 5 following ammonium exchange. Prior to ammonium exchange the Chabazites prepared with alkali metal hydroxides in the synthesis gel would be expected to contain >0.5 wt % Na2O.
The state of the art preparation of a Cu-Chabazite is described by the following key steps:                1. Crystallization of a alkali metal/SDA containing chabazite and separation from the synthesis gel        2. Drying and calcination to remove the SDA leading to the H—Na(alkali) form of Chabazite        3. Ammonium exchange to remove alkali metals        4. Copper exchange to introduce Cu        
Removal of alkali metals is important for the stability and activity of SCR catalysts. WO 2008/132452 suggests that the poor SCR performance of an alkali-metal containing CuChabazite could be attributed to poisoning of the acid sites and report little activity even in the fresh catalyst. Whereas, U.S. application Ser. No. 12/612,142 filed on Nov. 4, 2009 indicates good SCR performance of a Cu-Chabazite prepared from a similar parent material where the alkali-metals have been largely removed supporting the importance of low alkali-metal content.
Many catalytic uses for zeolitic materials involve the H-form and so step 2 of the inventive process already delivers an active material without the need for further processing. Such an application could include catalysts used in methanol to olefin chemistry.
Furthermore, the ion-exchange steps can lead to dealumination/deboronation due to the acidic pH conditions employed. The dealumination/deboronation limits the amount of active cations that can be introduced since it results in loss of exchange capacity and can lead to instability of the zeolite structure. Dealumination is linked to instability of SCR catalysts such as CuZSM-5 (Journal of Catalysis, 1996, p 43-).
Thus, the disadvantage of the multi-step synthesis route is the dealumination which can occur during ion-exchange steps. Additionally, each exchange step adds additional cost and additional complexity to the process. Partial replacement of expensive template molecules, such as trimethyladamantyl ammonium hydroxide, with tetramethylammonium hydroxide offers additional cost savings. The invention process results in a lower cost, less complex and less damaging synthesis route for the production of H-Chabazite and other metal containing forms of Chabazite.
Tetramethylammonium hydroxide (TMAOH) has been utilized as a templating agent and OH− source in numerous zeolite, zeolitic (e.g. borosilicate, gallosilicate etc) and non-zeolitic (i.e. AlPO, MeAPO, and MeAPSO compositions) syntheses including the preparation of ATT, CAN, CHA, EAB, ERI, ERI/OFF, FAU, FER, GIS, GME, LTA, MAZ, OFF, and SOD.
Barrer et al. discusses the role of OH− as a mineralizing agent together with the structure directing role of cations such as alkali metals and organic additives or templates (Zeolites, 1981, p 130). Control of both is critical for the selective crystallization of zeolite phases.
A number of Aluminophosphate materials can be crystallized using TMAOH including AlPO-12 (ATT—J. Phys. Chem., 1986, p 6122), AlPO-33 (ATT—U.S. Pat. No. 4,473,663), ZnAPSO-43 (GIS—EP 158,975), ALPO-20 (SOD—U.S. Pat. No. 4,310,440), BeAPSO-20 (SOD—U.S. Pat. No. 4,737,353), MgAPSO-20 (SOD—EP 158,348), MnAPSO-20 (SOD—EP 161,490) and ZnAPSO-20 (SOD—EP 158,975). These systems are synthesized in the absence of an alkali metal hydroxide since these materials typically crystallize in near neutral pH or less alkaline pH than the aluminosilicates materials. Consequently, these materials are considered alkali-metal free. Tetramethylammonium (TMA) is occluded within the microporous cavities of the material during crystallization.
The synthesis of the aluminosilicates ERI and OFF are described in many articles due to the overlapping synthesis conditions that often result in the intergrown product of the two known as ZSM-34. This complexity is comprehensively described in Zeolites, 1986, p 745. In all cases alkali metal hydroxides are used in combination with TMA. This paper represents the structures of ERI and OFF with the TMA cation occluded within the cages. The independent phases can be prepared by careful control of gel composition. Barrer et al. described the combination of TMA with alkali metal hydroxides for the preparation of CAN, LTA, OFF, ERI, EAB, GME, SOD and MAZ (Zeolites, 1981, p 130). The aluminosilciate, EAB crystallizes from a Na or K and TMA gel at temperatures of about 80° C. (J. Solid State Chem., 1981, p 204). In all cases the syntheses report a combination of TMA with an alkali metal resulting in the incorporation of both in the zeolite product.
Chabazite (zeolite ZK-14) with low SiO2:Al2O3 has also been reported to form with (K, Na, TMA) mixtures where K is preferred (Molec. Sieves, Soc. Chem. Ind., 1968, p 85). U.S. Pat. No. 4,544,538 teaches the synthesis of high silica chabazite from trimethyladamantylammonium hydroxide (TMAA) and sodium hydroxide reaction gels. It is mentioned that sodium hydroxide could be replaced by the addition of more template, whereas the template is typically a bicycle hetereoatom compound. It is disclosed that the preferred OH/Si ratio is <0.96 for the formation of chabazite with >20 SiO2:Al2O3. However, the addition of more template would result in a significant increase in cost and perhaps issues with waste water due to increased residual organic in the mother liquor following crystallization.
Zeolite RUT (Nu-1—U.S. Pat. No. 4,060,590 and RUB10, Z. Kristallogr., 1995, p 475) is formed from gels containing TMAOH with crystallization temperatures of 150 to 200° C. and reaction times of about 1.5 to 3 days. This is a common impurity phase when TMAOH is used as a replacement for alkali metal hydroxides in Chabazite synthesis due to similar reaction conditions. Increased amounts of TMAOH lead to RUT becoming the majority phase.
U.S. Pat. No. 3,306,922 describes a synthesis of zeolites N-X and N-Y (FAU), N-B and N-A (LTA) containing a substantial weight percent of a cation other than sodium or other metal cation. Specifically a low Na product is attained by using TMAOH as the only source of OH− and structure direction.
The prior art indicates that use of TMAOH, as the only organic source and in the absence of alkali-metals, would result in zeolites RUT, N-X, N-Y. N-B or N-A. No reports exist of Chabazite formation in the presence of only TMAOH.